In U.S. Pat. No. 3,842,194 issued on Oct. 15, 1974 to Jon K. Clemens, a video playback system of a variable capacitance form is disclosed. In one configuration of the Clemens system, information, representative of recorded picture and sound, is encoded in a relatively fine spiral groove on the surface of a disc record (e.g., groove width 3.5 micrometers, groove depth -- 1.0 micrometers). The record substrate is covered with a coating of conductive material and a dielectric layer overlying the conductive coating. During playback, a pickup stylus, having a thin conductive electrode (e.g., about 0.2 micrometers thick), engages the groove as the record is rotated by a supportive turntable. Capacitive variations between the stylus electrode and the conductive coating are sensed to recover the prerecorded information.
In the systems of the above type, the use of a relatively fine record groove and the groove-engaging requirement for the pickup stylus result in a stylus tip which is extremely small.
Typically, the stylus tip is defined by a prow, a substantially flat, V-shaped rear surface remote from the prow, a bottom surface extending from the bottom edge of the rear surface and a pair of side surfaces extending from the side edges of the rear surface and converging at the prow. The intersection of the rear and the side surfaces with the bottom surface forms, for example, a triangular footprint. The entire rear surface is covered with conductive material to form a V-shaped electrode. Illustratively, the angle included between the two side edges is equal to 42.degree., the length of the bottom edge is about 2 micrometers, the height of the triangular footprint is about 4 micrometers, the angle between the prow and the rear surface is about 40.degree., and the electrode depth is about 0.2 micrometers.
In the previously mentioned Clemens patent, two alternative forms of stylus structure are disclosed: (1) a symmetrical structure, wherein the conductive stylus electrode is "sandwiched" between dielectric support material, which extends ahead of, and behind, the electrode in symmetrical fashion (FIG. 1); and (2) an "asymmetrical" structure, wherein the stylus electrode comprises a conductive coating on the rear surface of a dielectric support (FIG. 5).
The asymmetrical stylus form has proven to be convenient for ease in stylus fabrication, in that the entire rear surface is coated with conductive material. However, the stylus electrode, being V-shaped, gets wider as the stylus structure wears away. As the stylus electrode gets wider, the spurious capacitance between the electrode and the conductive coating in the adjacent grooves becomes significant, introducing noise in the recovered signal. When the stylus electrode becomes wider than the groove width (e.g., 3.5 micrometers), it is desirable to replace the stylus.
Although the sandwich form of the stylus structure overcomes the problem of increasing cross-talk with the increase in the width of the stylus electrode, it is difficult to fabricate. In particular, it is difficult to achieve a good registration (i.e., position, attitude, etc.) between the stylus electrode and the stylus tip.
In the copending application, Ser. No. 782,020 of the applicants herein, entitled "NARROW-ELECTRODE PICKUP STYLUS FOR VIDEO DISC SYSTEM", filed concurrently herewith, a novel narrowed-electrode pickup stylus, which overcomes the above-mentioned problems, and yet, which is relatively easy to fabricate, is disclosed.
The narrowed-electrode pickup stylus is suitable for playing back prerecorded signals from a disc record groove of a given width. The pickup stylus comprises a dielectric support element which converges toward a tip. The dielectric support element has an arrowhead-shaped cross-section in the region of its tip. The arrowhead-shaped cross-section is defined by a prow, a substantially flat rear surface remote from the prow having substantially parallel edges, a pair of side surfaces converging toward the prow, a pair of additional side surfaces extending from the parallel edges of the rear surface and proceeding toward the prow, and shoulder surfaces interconnecting the converging surfaces and the additional surfaces.
The maximum distance between the substantially parallel edges of the rear surface, in the region of the stylus tip, is less than the given groove width.
The entire rear surface is covered with conductive material between its substantially parallel edges to form a narrowed-electrode in the region of its tip.